12 Key Points To Understand About Ladle Refractory Materials Part-1

 With the development of secondary refining, ladles have evolved from mere containers for molten steel to crucial metallurgical equipment for secondary refining. Consequently, the service conditions for refractory materials used in ladles have undergone significant changes, primarily manifested in the following aspects:

 The temperature of molten steel in secondary refining ladles has increased significantly, typically 50–100°C higher than that in ordinary ladles, and may exceed 150°C during the refining process. Higher temperatures significantly accelerate the erosion of refractories by molten slag and steel.

 The circulatory motion of molten steel in secondary refining ladles has intensified. Due to the adoption of technologies such as argon blowing, electromagnetic stirring, and vacuum treatment, the scouring and wear of refractories by molten steel have become severe.

 The corrosiveness of slag in secondary refining ladles has increased. Secondary refining furnaces require deep desulfurization and phosphorus slag smelting operations, resulting in high slag basicity and large slag volume, which significantly exacerbates the erosion of refractories.

 The molten steel holding time in secondary refining ladles has significantly prolonged, ranging from 1 to several times longer than that in ordinary ladles. As a result, the service life of refractories used in ladles has significantly shortened.

 In many cases, secondary refining ladles are operated under high-temperature vacuum conditions. Under vacuum, the evaporation rate of refractories accelerates, while their corrosion resistance also decreases.

 With the development of secondary refining technology for ladles, refractory materials used in ladles must meet the following requirements during their service:

 (1) High - temperature resistance: It should be able to withstand the long - term action of high - temperature molten steel without melting or softening.

 (2) Thermal shock resistance: It should be able to repeatedly withstand the charging and discharging of molten steel without cracking or spalling.

 (3) Resistance to slag erosion: It should be able to withstand the erosion of molten slag and the changes in slag basicity on the inner lining.

 (4) Sufficient high - temperature mechanical strength: It should be able to withstand the agitation and scouring of molten steel.

 (5) The inner lining has a certain degree of expansibility. Under the action of high - temperature molten steel, the inner linings are in close contact and form a whole.

 The working lining of a ladle is a crucial part that comes into contact with molten steel and slag. During its service, it is affected by the erosion, scouring, dissolution, and thermal shock damage from molten steel and slag. Therefore, the following conditions should be met:

 (1) In the construction process of the working lining, simple equipment and convenient construction should be strived for, which can reduce labor intensity and improve labor productivity. It should have good baking adaptability, reducing the energy consumption for baking the ladle, increasing the ladle utilization rate, extending the service cycle of the ladle, and reducing the number of standby ladles.

 (2) Under high - temperature service conditions, it should have good high - temperature properties. It not only requires a high refractoriness and certain high - temperature strength but also must have good chemical stability to ensure that there is no secondary oxidation of the molten steel, no contamination of the molten steel, and no reduction in the quality of the steel billet under high - temperature conditions.

 (3) During use, it should have good resistance to slag erosion and penetration, as well as the ability to withstand the scouring of molten steel and slag liquid, which is conducive to improving the service life of the ladle working lining, reducing the consumption of refractory materials for the ladle, and minimizing the contamination of the molten steel by refractory materials.

 (4) The working lining should have good thermal shock resistance and good volume stability, without cracking when in contact with molten steel, ensuring the good integrity of the ladle.

 (5) The working lining of the ladle should also have a low thermal conductivity and good heat - preservation performance, which can reduce the heat loss of the tundish and maintain the stability of the molten steel temperature in the tundish.

 (6) After use, the working lining should be easy to remove from the ladle. The working layer and the permanent layer should be easily separated, which can reduce the damage of the refractory materials of the working lining to the permanent lining of the ladle and help extend the service life of the ladle.

 Ladle turnover process: Steel tapping from converter/EAF → secondary refining treatment → continuous casting → ladle preparation → waiting for tapping. The normal turnover time varies depending on the steel grade and continuous casting machine, ranging from 100 to 140 minutes. The tapping temperature is 1680–1700°C, and the molten steel holding time is 100–120 minutes. Typical ladle slag composition (%) for full continuous casting operations: Al₂O₃ 17%–26%, SiO₂ 8%–10%, CaO 42%–47%, MgO 5%–11%, FeO 18%–22%. For ultra-low carbon steel processes such as silicon steel, bridge steel, and automotive panel steel, vacuum treatment is mandatory, combined with argon blowing at the ladle bottom for stirring, and LF furnace operations including arc heating, reducing atmosphere, white slag refining, gas stirring, etc. These enhance thermodynamics and kinetics conditions, comprehensive refining effects such as desulfurization, alloying, and temperature rise. As a result, the slag basicity range is wide, the temperatures of molten steel and slag are higher, the residence time of molten steel in the ladle is prolonged, thermal shock is intense, and stirring force is strong, exacerbating damage to the ladle lining.

 The causes of damage are as follows:

 First, ladles transport high-temperature molten steel. During transportation, molten steel at approximately 1680°C and slag erode and scour the lining, particularly the slag line area, which undergoes severe erosion and is a critical factor determining the ladle's service life.

 Second, out-of-furnace refining treatments like LF cause significant damage to unburned bricks.

 Third, during converter tapping and steel flow, the lining withstands drastic temperature changes, leading to cracks and spalling in the lining materials.

 Fourth, when molten steel is charged into the ladle from the converter, the high-temperature steel causes intense mechanical scouring of the bottom, making the lining materials in this area prone to damage from thermal shock.

 During the service of refractory materials, molten slag easily penetrates from the heated surface into the inner depths, significantly reducing the porosity near the working surface and densifying it, forming a thick metamorphic layer. When the temperature changes drastically, cracks parallel to the working surface occur at the junction between the metamorphic layer and the original brick layer, causing the brick to spall and deteriorate.

 To reduce the structural spalling of refractory materials, the key is to minimize the penetration depth of slag, which can be achieved through the following approaches:

 (1) Improve the slag penetration resistance of refractory materials;

 (2) Reduce the porosity of refractory materials to decrease the erosion channels for slag;

 (3) Form high-melting-point compound barriers through the reaction between slag and refractory materials to prevent slag penetration;

 (4) Increase the viscosity of the slag-the higher the slag viscosity, the lower its erosion capacity on refractory materials.

 Ladle purge bricks are the most critical functional components in the secondary refining process, and their main functions are as follows:

 (1) They can regulate the uniform distribution of molten steel temperature in the ladle to achieve the optimal casting temperature for the existing process.

 (2) Through gas blowing and stirring, alloys and deoxidizers in the ladle can be uniformly distributed.

 (3) They can carry non-metallic inclusions in the molten steel into the slag to meet the cleanliness requirements of the molten steel.

 To achieve the above functions, inert gases used in refining need to be blown into the ladle through the purge bricks. On the working surface of the purge brick (i.e., the contact surface between the purge brick and the molten steel), a large number of bubbles blown out under sufficient pressure form a gas injection beam, which stirs the molten steel in the entire ladle, promotes the flow of molten steel, and homogenizes the temperature and composition in the ladle. At the same time, the continuously ejected bubbles carry non-metallic inclusions in the molten steel into the slag through interfacial action, achieving the purpose of cleaning the molten steel.